Radome and method for making same



March ll, 1969 T. J. JORDAN ETAL 3,432,859

RADOME AND METHOD FOR MAKING SAME Filed Jan. 29, 1955 Sheet March 11,1969 12.1. JORDAN ETAL 3,432,359

RADOME AND METHOD FOR MAKING SAME Filed Jan. 29, 1963 Sheet 2 of 2 Q97Z1 4. m

The/k ACCO/Wgg United States Patent Office 3,432,859 Patented Mar. 11,1969 RADOME AND METHOD FOR MAKING SAME Thomas J. Jordan, Ballston Lake,and Henry T. Plant,

Schenectady, N.Y., assignors to General Electric Company, a corporationof New York Filed Jan. 29, 1963, Ser. No. 254,798

US. Cl. 343-872 Int. Cl. H01q 1/42, 15/08; E04b 1/32 The presentinvention relates to dielectric, microwavetransparent, panels and, moreparticularly, to radomes formed with such panels having high strengthand rigidity coupled with a uniform dielectric constant.

For various applications, radomes are generally required to exhibit acombination of very low, uniform dielectric constant, minimum weight,high strength, and rigidity. Present materials and techniques for makingradomes do not satisfy these requirements, especially with regard touniformity of the dielectric constant.

It is quite common today to construct radomes by laminating severallayers of materials having different properties, for example, dielectriccharacteristics and mechanical strength, in order to take advantage ofthe various properties. Such constructions are relatively pervious toelectromagnetic radiation but are usually limited to a rather narrowrange of frequencies. Additionally, such constructions are onlyefficient when the radiation angles of incidence vary within narrow, lowangles. These limitations on the performance of conventional laminatedradomes create serious disadvantages in broad-band applications wherewide frequencies and incident angles are generally encountered.

In radome applications requiring weight minimization without sacrificeof strength, sandwich construction has been resorted to, wherein, thesandwich core is usually either a honeycomb construction or a foamedplastic. In order to achieve uniform dielectric properties, the skinelements of a sandwich must be spaced apart accurately and be of uniformthickness. In addition, the core must be homogeneous. The skins areoften cemented to the core, thus, introducing an extraneous substancewhich often has adverse effects on transmitted microwavecharacteristics.

Accordingly, it is an object of this invention to provide a microwavetransparent panel having a uniform, low dielectric constant along Withlow weight, high strength, and rigidity.

Another object of this invention is to provide a microwave transparentpanel which efficiently transmits electromagnetic radiation over a widerange of frequencies and incidence angles.

A further object of this invention is to provide a method forfabricating microwave transparent panels havmg a uniform, low dielectricconstant along with low weight, high strength, and rigidity.

A still further object of this invention is to provide a method forfabricating microwave transparent panels wherein the dielectricproperties of the panel can be varied and closely controlled.

Further objects and advantages of this invention will be betterunderstood from the following description taken 1n connection with theaccompanying drawings. The features of novelty which characterize thisinvention will be pointed out with particularity in the claims annexedto and forming part of this specification.

In the drawings, FIGURE 1 is a perspective view showing a final radomeconfiguration incorporating the principles of this invention.

FIGURE 2 is a magnified perspective view showing a panel constructionsuitable for a radome which comprises a plurality of layers ofcorrugated sheets, the corrugation 9 Claims axes of adjacent sheetsbeing angularly disposed with respect to one another.

FIGURE 3 is a magnified perspective view depicting a different panelconstruction suitable for transmission of microwave energy therethroughwhich comprises a plurality of layers of corrugated sheets, thecorrugation axes of adjacent sheets being parallel to one another.

FIGURE 4 shows a magnified cross-sectional view showing a panelconstruction comprising a plurality of layers of alternating corrugatedand planar sheets.

FIGURE 5 is a magnified perspective view showing a panel constructioncomprising a plurality of layers of sheets having embossments formedtherein.

FIGURE 6 is a magnified perspective view showing a panel constructioncomprising a plurality of layers of hollow, cylindrical elements, thelongitudinal axes of the cylindrical elements of all layers beingparallel to one another and the elements within a single layer having acommon transverse axis.

FIGURE 7 is the same as FIGURE 6 except the longitudinal axes ofelements in one layer are angularly disposed with respect to thelongitudinal axes of elements in an adjacent layer.

In accordance with the various embodiments of this invention, a rigidpanel having a uniformly low dielectric constant and being of lowweight, high strength, and rigidity is produced by fabricating asimulated foam of plastic material having a cellular structure withcontrolled, welldefined air cells therein. The simulated foam is formedby laminating layers of plastic material such that the assembly definesa plurality of elongated passages therethrough. This structure is formedwith alternations of air and plastic material such that, by propergeometrical design, the desired dielectric characteristics and densityis attained.

Referring to the drawings, radome 1 (FIG. 1) may be fabricated fromdielectric, transparent, microwave panel 2 (FIG. 2) having satisfactorymechanical and electrical properties. In this embodiment, a plurality ofthin sheets 3 of plastic material are corrugated and laminated so thatcorrugation axes of adjacent sheets are angularly disposed with respectto one another.

In order to satisfy the strict requirements necessary for propermicrowave transparency, the most important and critical step in theformation of a radome panel is production of the individual sheets 3from which the panel is formed. Close control of the thickness dimensionand uniformity is required. Starting with a homogeneous plastic materialhaving the desired dielectric properties, it is necessary to produce aplurality of sheets 3 having a minimal thickness which, when laminated,will maintain a minimum dielectric constant. Extrusion to a minimalthickness of about .006 inch with a tolerance of about .0005 inch hasbeen successful. After extruding the flat sheets, the subsequentcorrugating step must also be closely monitored, such as by vacuumforming. It has been found that, in order to avoid deleteriousreflections, corrugation heights should not exceed one-tenth of theradiation wavelength to be used.

The step after corrugation is lamination which is defined as thestacking and bonding of a plurality of layers, the layers consisting ofplanar of preformed sheets or a plurality of elements unitarilyassembled within a single plane, such as a plurality of cylinders to bediscussed below. By careful maintenance of the quality of material usedand careful formation of the corrugated sheets, controlled uniformelectrical and mechanical properties are precisely defined. This highdegree of uniformity is readily attainable through many manufacturingprocesses in common use today.

The materials of construction for the layer elements are known asplastics and provide the high strength qualities needed. The termplastics encompasses synthetic organic resins and ceramics, includingthose which are reinforced. Ceramic and reinforced ceramic materials areparticularly applicable for radomes where high temperatures or radiationare likely to be encountered.

In the laminating process for the thermoplastic sheets it has been foundthat bonding of the adjacent layers is best accomplished by solventwelding which does not introduce permanently any extraneous materialadversely affecting the electrical properties of the laminated product.

FIGURE 3 illustrates an alternate panel construction wherein adjacentcorrugated layers 4 are arranged with the axes of corrugations parallelto one another. The corrugated sheets. are arranged such that they donot nest within one another but instead are assembled so thatcorrugations between adjacent sheets define a plurality of elongatedpassages 5.

FIGURE 4 illustrates a similar panel construction utilizing corrugatedsheets 6 oriented with parallel corrugation axes. In this embodiment,planar or flat sheets 7 are interposed between the corrugated sheets 6to define elongated passages 8 between the surfaces of adjacent sheets.Obviously, the present, embodiment is merely illustrative and notexhaustive of various combinations and formations that can be achievedwith corrugated and non-corrugated sheets.

An alternate configuration of the panel construction is illustrated inFIGURE 5. The individual layers for the composite member may be preparedby formation of a plurality of embossments 9 in thin sheets 10 ofplastic material. The embossments 9 may be of any conceivable shape suchas hemispherical, rectangular, pyramidal, etc. The embossed sheets 10are then laminated such the the embossments are staggered in positionand are contiguous with an embossed portion of an adjacent sheet. Theembossments serve as spacers or short columns defining passages 14between adjacent sheets. As with the embodiments using corrugatedsheets, layers of embossed sheets may be alternated with layers ofplanar sheets to achieve similar results.

Another alternative configuration employs a plurality of hollowcylindrical elements 12 as illustrated in FIG- UR-ES 6 and 7. Thecylindrical elements are arranged in layers such that the longitudinalaxes of cylinders in each layer are parallel with the transverse axe's13 of all cylinders in each layer being within a single plane, albeitnot necessarily a fiat plane. For example, the layers may be formed intoa warped plane of the shape finally desired.

Successive layers of the cylinders may be laminated having longitudinalaxes parallel with respect to one another as illustrated in FIGURE 6 ortransverse to one another as illustrated in FIGURE 7. The relative anglebetween the axes may be of any value between the extremes illustrated inFIGURES 6 and 7 such as 30 or 45 degrees.

As was described for the above embodiments utilizing corrugated sheets,the embossed sheets and layers of cylinders are preferably bondedtogether by solvent welding. It should also be noted that for any of thedescribed embodiments, a single unitary panel may be fabricated toobviate the joining together of a plurality of smaller panels. Inaddition, the final member may be flat or formed into various shapes asmay be required in the particular radome application.

While specific forms and methods of the invention have been shown anddescribed, it will be apparent to those skilled in the art that numerouschanges, combinations and substitutions of equivalents might be made. Itis therefore contemplated by the claims which conclude the specificationto cover all such modifications as fall within the true spirit and scopeof this invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. A rigid panel comprising a radome through which microwave energy istransmitted having substantially uni' form dielectric properties, saidpanel comprising a plurality of layers of preformed elements, each ofsaid elements being made of plastic material and having a uniformthickness and uniform dielectric properties, said elements beingpreformed such that, upon assembly thereof into layers, a plurality ofelongated passages is defined, the maximum dimension of said passages ina direction parallel to said transmitted microwave energy being notgreater than one-tenth of the wavelength of said microwave energy.

2. A rigid radome as defined in claim 1 wherein said elements are hollowcylindrical bodies and wherein each of said layers comprises a pluralityof the cylinders oriented to have parallel longitudinal axes and acommon transverse axis, said longitudinal axes being substantiallyperpendicular to the direction in which said microwave energy istransmitted and wherein the hollow interiors of said cylindrical bodiesconstitute said elongated passages.

3. A rigid radome as defined in claim 2 wherein the longitudinal axes ofthe cylinders in each layer are parallel to the longitudinal axes of thecylinders in an adjacent layer.

'4. A rigid radome as defined in claim 2 wherein the longitudinal axesof the cylinders in each layer are angularly disposed with respect tothe longitudinal axes of the cylinders in an adjacent layer.

5. A rigid radome through which microwave energy is transmitted havingsubstantially uniform dielectric properties, said radome comprising alamination of a plurality of sheets having corrugations formed therein,each of said sheets being made of plastic material and having uniformthickness and uniform dielectric properties, said corrugations defininga plurality of elongated passages, the maximum height of saidcorrugations being one-tenth of the wavelength of said microwaves.

6. A rigid radome as defined in claim 5 including a plurality of planarsheets being made of plastic material and having uniform thickness anduniform dielectric properties, said lamination comprising alternationsof said corrugated sheets and said planar sheets.

7. A rigid radome through which microwave energy is transmitted havingsubstantially uniform dielectric properties, said radome comprising alamination of a plurality of sheets having a plurality of embossmentsformed therein, each of said sheets being made of plastic material andhaving uniform thickness and uniform dielectric properties, said sheetsbeing assembled such that the embossments of one sheet are contiguouswith and about an unembossed portion of an adjacent sheet thus defininga plurality of elongated passages between adjacent sheets, the maximumdimension of said passages in a direction parallel to said transmittedmicrowave energy being not greater than one-tenth of the wavelength ofsaid microwaves.

8. A method for making a rigid panel through which microwave energy istransmitted having substantially uniform dielectric propertiescomprising thesteps of:

(a) providing a plurality of thin sheets of plastic material, each ofthe sheets having uniform thickness and uniform dielectric properties,

(b) offsetting portions of the sheets to form projections therefrom and,

(c) laminating the sheets such that adjacent sheets and the projectionstherefrom define a plurality of elongated passages each having a maximumdimension in a direction parallel to the direction in which saidmicrowave energy is transmitted of not more than one-tenth thewavelength of the microwaves.

9. A method as defined in claim 8 wherein the step of offsettingcomprises corrugating the sheets.

(References on following page) References Cited UNITED STATES PATENTSWatson 156-591 Ruppricht 161-136 Shipley et a1. 161-6'8 Wentworth et a1.156-205 Pajak 156-207 Lincoln 16 1- 69 Meyer 161-137 X Overholt 161-89Colman et a1 343-872 X 6 2,045,849 6/1936 Genter 161-69 3,041,2236/1962, Sage 161-69 2,751,964 6/ 1956 Guyer 156-205 EARL M. BERGERT,Primary Examiner.

H. F EPSTEIN, Assistant Examiner.

U.S.C1.X.R.

1. A RIGID PANEL COMPRISING A RADOME THROUGH WHICH MICROWAVE ENERGY ISTRANSMITTED HAVING SUBSTANTIALLY UNIFORM DIELECTRIC PROPERTIES, SAIDPANEL COMPRISING A PLURALITY OF LAYERS OF PREFORMED ELEMENTS, EACH OFSAID ELEMENTS BEING MADE OF PLASTIC MATERIAL AND HAVING A UNIFORMTHICKNESS AND UNIFORM DIELECTRIC PROPERTIES, SAID ELEMENTS BEINGPREFORMED SUCH THAT, UPON ASSEMBLY THEREOF INTO LAYERS, A PLURALITY OFELONGATED PASSAGES IS DEFINED, THE MAXIMUM DIMENSION OF SAID PASSAGES INA DIRECTION PARALLEL TO SAID TRANSMITTED MICROWAVE ENERGY BEING NOTGREATER